The pieces of the puzzle

In the late 1400s, the great Leonardo stood at the foot of a cliff, looked at the layers, and scratched his head. There were shells, so it had to be something to do with the sea. But the layers, all those layers … how come?

Layer upon layer upon layer. Layers within layers. Some thick, some thin. Some hard, some soft. Some one color, some another. The interfaces sometimes bold, sometimes subtle. But no obvious order. How come indeed?

In the 1670s Steno confirmed the marine origin of many layers, and by the 1760s Lavoisier had distinguished between layers formed during transgressions and regressions of the sea. The stage was set for Hutton and Smith and the great geologists of the 1800s, who gradually sorted it all out. But not without a lot of thrust and parry, of ding and dong. And a few loose ends … like order, and bedding planes.

Coal miners were the first to spot that rock successions sometimes repeat. When they talked, Smith listened. Later, geologists saw that some examples were caused “in plan,” as when a channel changed course, and others vertically, by repetition of a tectonic event or a sea-level change.

The advent of photography helped. Now one could pin the outcrop on the wall, and scratch the head in comfort. But there was a new puzzle: How far away was this photo taken? Close up or far away—the layers often look the same.

A geologist of the day took his girlfriend on a field trip. “Why are you always taking photographs of boring old rocks,” pouted Hermione, “when you could be taking photographs of me?” Good idea. From then on, Hermione appeared in photograph after photograph. Always against the rock, always at the edge of the picture … “for …